1. Field of the Invention
The present invention is applicable to telephone instruments and more particularly to telephone instruments which require battery backup for memory retention and microprocessor operation during zero line power conditions or during loop signaling. The invention may also find applicability as battery backup power for computer systems or security systems.
2. Background Art
The basic circuitry used in many prior microprocessor based telephones powered by the telephone subscriber's line or a backup battery supply is like that shown in FIG. 1 of the attached drawings. As shown diode isolation is used to decouple the battery supply (B) from the V.sub.DD supply whenever the V.sub.DD voltage supply exceeds the battery supply voltage. This increased V.sub.DD supply voltage represents a condition where sufficient power can be obtained from the telephone subscriber line. Telephones employing such circuitry include the "Duofone 160" marketed by Radio Shack Incorporated and the Electronic Resources Ltd., type IT2005 telephone. A similar technique is used to power a combination repertory dialer within the "Linear II" telephone manufactured by GTE Communication Systems Corporation and described in U.S. patent application Ser. No. 558,128 by E. Boeckmann, entitled "Protection Circuitry for Subscriber's Telephone Circuit Equipped with Repertory Memory". A similar technique is also used on the +5 volt DC regulated voltage supply, powered from a 110 volt AC power line source for the random access memory (RAM) included in the "Featurephone" also manufactured by GTE Communication Systems Corporation. In this instance, a 3.6 volt battery is used to maintain memory during AC power failures.
The prior art does not appear to include a repertory dialing telephone which maintains the same regulated DC supply voltage regardless of the energy source. The aforementioned circuits exhibit a characteristic voltage difference when the energy source has changed from a backup battery to the external AC power line or to the telephone subscriber line. Such an event can have a degrading effect upon the performance of multiplexed liquid crystal displays (LCD's) which may be incorporated in such telephones. Normally the contrast exhibited by these displays is maintained by a temperature compensated voltage drop between the DC voltage supply and the voltage supply inputs to the integrated circuit LCD driver. Such a compensating voltage drop may include an adjustable resistor and a thermistor as described for the HLCD 0515, LCD driver in the 1982 CMOS data book for Hughes Solid State Products. As a result, this change in the DC voltage supply is reflected by a corresponding change in the AC drive signals to the LCD display and a subsequent change in the contrast of the display.